Due to trends toward weight reduction and higher performance of automobiles, springs have been strengthened and high strength steels having a tensile strength of more than 1,600 MPa after a heat treatment have been applied to springs. In recent years, steels having a tensile strength of more than 1,900 MPa have also been used.
Methods for producing a coil spring using a steel include a hot coiling process comprising heating the steel to the austenite region for coiling, and then quenching and tempering the steel; and a cold coiling process comprising cold coiling a high strength steel wire made of the steel quenched and tempered in advance. In both cases, the fundamental strength of the spring is determined by quenching and tempering. Therefore, composition design considering characteristics after quenching and tempering is important for a spring steel.
For example, like Patent Documents 1 to 3, for the purpose of strengthening, a large amount of C is basically added and also alloying elements such as V and Mo are added to thereby improve hardenability and temper softening resistance.
Like Patent Document 4, surface hardening due to a nitriding treatment is effective so as to further strengthen a spring. Usually, the nitriding treatment is applied after coiling of the spring. Since this treatment is performed by heating at 400 to 600° C., a surface of the spring is hardened, while a core portion is softened and spring performances such as fatigue characteristics may conversely deteriorate if the core portion does not have a sufficient softening resistance. Therefore, alloying elements capable of imparting temper softening resistance are commonly added.
However, even if alloying elements capable of imparting temper softening resistance are added, the fatigue strength is not increased by enhancing temper softening resistance so as to increase the strength if hard inclusions such as SiO2 exist in the steel, or a decarburized layer exists on a surface layer.
For example, in Patent Document 5, a slag composition in a molten steel treatment is controlled in an appropriate range to thereby enhance ductility of inclusions which may cause a decrease in fatigue strength, and also inclusions are refined by hot rolling to thereby increase the fatigue strength.
For example, in Patent Document 6, heating conditions before hot rolling and cooling conditions after rolling are appropriately controlled, and scales on a surface are removed before hot rolling to thereby suppress the formation of a decarburized layer. It has recently been required for springs used in automobiles to have more increased strength. The fact is, however, that conventional steels for high strength spring cannot satisfy such requirements.
In addition, Patent Document 7 describes a hot rolled wire rod usable as a raw material of drawn wire products such as a spring steel, which is excellent in wire drawability and also can suppress wire breakage even in the case of heavy drawing from a thick wire rod. Patent Document 8 discloses a steel wire for cold formed spring, which is excellent in cold cuttability and fatigue characteristics.